1. Field of the Invention
This invention relates to apparatus for irradiating a specimen by an electron beam. More particularly, it relates to the apparatus suitable for use in an electron microscope, an electron probe X-ray microanalyzer, an Auger electron analyzing device and an electron-beam pattern drawing device.
2. Description of the Prior Art
In apparatus for irradiating a specimen by an electron beam, such as electron microscope, electron probe X-ray microanalyzer, Auger electron analyzing device and electron-beam pattern drawing device, it is important that the specimen is saved from staining. Although causes for the staining of the specimen are complicated, the main cause is deemed to lie in that hydrocarbon molecules adsorbed to the specimen form a layer which is stable both thermally and chemically, on the specimen in virtue of the irradiation by the electron beam. In order to prevent such stains of the specimen, attention has heretofore been turned to that a specimen chamber in which the specimen is accommodated is evacuated to the highest possible vacuum. From this viewpoint, a high vacuum pump such as oil diffusion pump and sputter ion pump is ordinarily used for maintaining the steady vacuum of the specimen chamber. These techniques are respectively disclosed in Japanese Pat. Publications Nos. 3224/74 and 48945/76.
However, oil vapor flows backwards into the specimen chamber from the oil diffusion pump. In substance, accordingly, the use of the oil diffusion pump does not serve to prevent the staining of the specimen. The oil diffusion pump also has the problem that it takes as long a time as about 20 - 30 minutes to steadily operate. When a trap for the oil vapor is interposed between the oil diffusion pump and the specimen chamber, the backward flow of the oil vapor into the specimen chamber is relieved, but the effect of preventing the backward flow is still unsatisfactory. In the case where the sputter ion pump is employed as the main evacuating pump, a higher degree of vacuum can be reached. On the other hand, however, it becomes a cause for the stains of the specimen that sputter substances produced within the pump adhere (deposit) onto the specimen. The adhesion of the sputter substances onto the specimen becomes more conspicuous as the sputter ion pump is brought nearer to the specimen for the purpose of keeping the vicinity of the specimen at a higher vacuum. Another problem is that ions and electrons produced within the sputter ion pump are captured as false signals by a detector for detecting information signals (of, for example, secondary electrons, reflected electrons, X-rays, cathode luminescence, Auger electrons, etc.) from the specimen.